In conventional techniques, an elastic blade method has been used as a toner layer forming method in nonmagnetic one-component developing (developing by using a nonmagnetic toner only but not a magnetic carrier) devices in electrophotographic devices such as printers, copiers and facsimile machines in which images are formed by an electrophotographic method. An elastic blade method means forming a toner layer by urging the surface of the elastic blade to contact with a toner holding member. An advantage of this method is cost reduction due to the simple configuration, and stable toner thin layers can be formed with relative ease.
A problem in this method is, however, that toner will be adhered readily on the elastic blade that is a toner layer control surface since the elastic blade is fixed on the toner holding member, and thus, stability cannot be obtained over the course of time. In order to solve the problem, a rotary type toner layer forming method was suggested. In this method the toner control surface keeps changing relative to the toner holding member.
In this method, toner thin layers are formed by contacting the surface of a self-rotating roller-type toner layer control member (hereinafter, a rotary toner layer control member) with the surface of the toner holding member. Publication of Japanese Patent Application (Tokkai-Hei) No. 1-62675 discloses a toner layer forming method in which layers are formed stably by determining the peripheral speed of the rotary toner layer control member. Publication of Tokkai-Hei No. 3-81788 discloses another toner layer forming method for providing stable toner layers by applying d.c. bias and superimposing a.c. bias to a rotary toner layer control member in a rotary toner layer forming method.
In an ordinary rotary type toner layer forming method, a toner removing member (a scraper) for scraping off the toner on the rotary toner layer control member is urged into contact with the rotary toner layer control member. Publication of Tokkai-Hei 2-190877 discloses that the toner on a rotary toner layer control member is removed smoothly by determining the strength of the materials of the toner removing member and that of the rotary toner layer control member, resulting in a stable removing (scraping) property.
The nonmagnetic one-component development using such a conventional rotary toner layer forming method, however, will cause the following problems.
(1) In order to form stable toner layers for a long time in a rotary toner layer forming method, the rotary toner layer control member should be located so that its surface with no toner adhesion is always facing to the toner holding member.
For this purpose, the toner adhered to the rotary toner layer control member should be removed constantly by the toner removing member that is urged into contact with the rotary toner layer control member.
However, when the toner removing member deteriorates over time, a so-called scraping failure will occur, that is, the toner cannot be removed completely. Once a scraping failure occurs, a linear image failure is generated on a filial image (the toner image obtained on a recording material after transcription from a photoconductor).
When using a black toner containing carbon black, such a scraping failure merely occurs, because the carbon black adhering to the toner removing member functions as a lubricant, so the toner will be merely accumulated on the toner removing member. However, scraping failures occur frequently when using a color toner that does not contain carbon black and that uses a sharpmelt resin (a resin with low softening point and low melting viscosity) for a binder resin, resulting in serious problems.
(2) When a.c. bias is applied between the rotary toner layer control member and the toner holding member in order to form a toner layer stably, leakage of the ac. bias occurs and the toner layer formation becomes uneven. In a worst case, the toner is heated and melted by the excess current flowing in the toner and is adhered to the peripheral members (e.g., the toner holding member and the photoconductor), or the image unevenness is generated on the final image.
Such failures will often occur specifically when the toner holding member has low volume resistivity. Furthermore, the frequency of such problems caused by the a.c. bias leakage is extremely high compared to those of an elastic blade type device.